Various methods for manufacturing printed circuit boards are known. An overview is given, for example, in the following publications: "Printed Circuits Handbook", C. F. Coombs, Jr. (ed.), McGraw-Hill 1988; and "Handbuch der Leiterplattentechnik", Gunther Herrmann, Eugen G. Leuze Verlag, 1982 EP-A2-O150733.
The print and etch method is used particularly for producing one-sided printed circuit boards wherein the conductive paths are applied only on one side of the board, for example, printed circuits with low packing density. The print and etch method begins with a board of non-conductive material with a copper layer laminated to one or both sides of the board. The conductive structures of the printed circuit board may be applied by using, for example, screen-printing or photographic methods (applying photoresist, exposing, developing).
For manufacturing two-sided printed circuit boards or multilayer printed circuits, one of the following methods can be used: subtractive method, fully-additive method, or semi-additive method.
The subtractive method begins with a copper-laminated isolating carrier plate, for example a glass-fibre reinforced epoxy resin plate with a copper foil on both sides of the plate having a typical thickness of 35 micrometers. Inner layers of multilayer circuits are produced by first applying a positive resist to the board. Then, the resist is exposed to light according to the pattern of conductive structures to be produced on the board and thereafter developed. Then, uncovered copper is etched away, the resist is removed and the surface conductive paths are oxidized to form a protective layer. Finally, the thus produced inner layers are pressed together to a package such as to form a multilayer circuit.
In order to produce inner layers having through-holes, the outer layers of multilayer circuits and two-sided circuits developed according to the subtractive method require certain additional process steps. First, the holes through which electrical contacts are to be established are drilled. The walls of the through-holes are metallized by chemical deposition of copper. Then the metallized walls of the through-holes are reinforced. The conductive paths are formed using the panel or pattern plating method. Finally, the outer layers are covered with a solder stop.
The subtractive method has the disadvantage that the largest part of the comparatively thick copper foil has to be etched away for the production of the conductive paths thus causing considerable loss of copper. Furthermore, since under-etching of the conductive paths cannot be avoided, the subtractive method is limited to the production of conductive structures having certain dimensions i.e., width and/or spacing of the conductive paths, of 80 to 100 micrometers.
The fully-additive method for manufacturing printed circuit boards differs from the subtractive method in that the starting board of isolating material is not covered with a copper foil, but is either a catalytic base laminate or is covered with an adhesive. After drilling the through-holes and application of a resist, the sleeves of the through-holes and the conductive paths are formed by chemical deposition of copper.
In the semi-additive method, the starting material is the same as that used with the fully-additive method. After drilling the through-holes, however, the entire surface of the board is covered with a thin layer of chemically deposited copper. Thereafter, a negative resist is applied and the walls of the through-holes and the conductive structures are electroplated. After removing the resist, the thin copper layer between the conductive structures is etched away.
As compared to the fully additive method, the semi-additive method has the advantage that the metallic sleeves in the through-holes consist substantially of electroplated copper which has an increased ductility. The disadvantages is the greater number of process steps. A disadvantage of the fully-additive and the semi-additive method in view of the subtractive method is that the adhesion of the conductor structures on the board of isolating material is worse in the additive methods. An advantage of the additive methods in view of the subtractive method is that the latter method permits in principle the production of finer conductor structures.
In United Kingdom Patent Number 1,056,814, a method for making printed circuit boards with conductive passageways through the insulating carrier board is shown. In this method, the walls of the through-holes are made conductive exclusively by electroless metal deposition. The method includes a "sensitizing" step wherein at least the walls are treated with a sensitizing solution in order to ensure good adherence of the metal to the walls, during the electroless metal deposition on the walls of the through-holes. A layer of metal is also deposited on the conductive traces on the surface of the isolating carrier board to render such traces rugged.